Polypeptide factor from a thermophilic eubacterial species and use thereof in the production of functional, heterologous proteins in an expression host

ABSTRACT

A polypeptide factor derived from the thermophilic eubacterial species  Thermus thermophilus  has universal protein expression-assisting activity. The polypeptide factor has been named the CzrB protein active in full length or truncated form has the potential to act as a universal protein expression-assisting molecule which can increase the yields of all heterologous proteins produced in  E. coli  by a mechanism that is independent of the protein being expressed.

FIELD OF THE INVENTION

[0001] This invention relates to the production of heterologous proteins in a host organism and to protein expression-assisting molecules which result in said heterologous proteins having functional activity, due to a correct folding thereof.

BACKGROUND AND PRIOR ART

[0002] The production of heterologous proteins in bacterial hosts such as the bacterium Escherichia coli (hereinafter referred to collectively as E. coli and exemplified by E. coli except where otherwise expressly stated) is a powerful tool in the generation of many important biotechnological and medical products. This technique involves inserting the DNA encoding the product in question into an E. coli cell and using the cell to convert the genetic information into a functional protein.

[0003] Research over the past 20 years has demonstrated the ability of E. coli to serve as the expression host for a wide variety of proteins from numerous sources, ranging from other Gram-negative bacteria to mammalian proteins.

[0004] Improvements to the basic technology include the development of secretion mechanisms, whereby polypeptides are exported to the periplasmic space or the extracellular medium, as required for their folding and/or activity. The periplasmic space is of particular interest to the biotechnologist in terms of heterologous protein production in E. coli, due to its oxidising environment. This allows the formation of disulfide bonds, which are essential for correct folding and activity of many mammalian proteins of medicinal and/or biotechnological interest, such as antibody molecules.

[0005] Other improvements to fundamental expression systems in E. coli include greater control of expression of heterologous proteins, and novel peptide tags encoded on expressed molecules to facilitate their detection and purification.

[0006] Due to its extensive genetic and biochemical characterisation, E. coli is frequently the organism of choice for heterologous protein production experiments. E. coli also exhibits simple fermentation pathways and has a short doubling time, which is also advantageous. Furthermore, the nutritional (and sterility) requirements of E. coli are uncomplicated, relative to higher organisms.

[0007] A major disadvantage of E. coli as an expression host, however, is the fact that the yields attainable with this organism are relatively low, while it frequently also exhibits difficulties in synthesising proteins derived from eucaryotic sources. These difficulties can take the shape of an inability to carry out particular post-translational modifications of the translated polypeptide or, more fundamentally, an inability of the E. coli cellular machinery to fold the peptide in the first place. In instances such as the latter, the available solutions have been to translate the polypeptide in E. coli, followed by refolding in vitro—a time-consuming and highly inefficient process—or to switch expression host to a higher organism which can carry out the expression efficiently, but with the concomitant loss of advantages of E. coli, as outlined above.

[0008] While E. coli carries out the process of gene expression and protein production very efficiently with its own, natural proteins, it is considerably less productive when expressing proteins from other species. This is most likely due to an inability to correctly fold the translated polypeptide, or to successfully transport it to the appropriate subcellular compartment for assembly or folding. Such a deficiency may result from the E. coli “synthetic machinery” being unable to recognise or act upon heterologous proteins due to differences in such proteins relative to E. coli 's native proteins. Alternatively, it may reflect an inability on the part of the host cells to express genes at the high levels demanded in such biotechnological experiments due to saturation of its normal gene expression and/or protein synthetic machinery. In such a scenario, the expressed protein typically forms large, insoluble aggregates consisting of multiple copies of the protein, which is non-functional and may be destroyed by the normal cellular machinery.

[0009] Furthermore, expression of heterologous genes in E. coli appears to frequently subject the cells to severe stress, leading to damage to the outer membrane of the host E. coli cell and leaking of the contents of the cell into the culture medium. This is typically followed by cell death via lysis of the E. coli cells.

[0010] This response of E. coli to expression of foreign genes has important implications for its potential in the production of a wide variety of heterologous proteins. With some foreign genes, E. coli has been found to be incapable of producing any functional protein; in cases in which E. coli folds the translated protein inefficiently or is overly stressed as a result of its expression, yields of the heterologous protein are dramatically reduced.

[0011] Researchers have attempted to overcome these problems with heterologous protein production in three main ways: i) genetic modification of the protein being expressed to improve its production in E. coli; ii) manipulation of the growth environment in order to reduce the stress on the expressing bacteria; and iii) co-expression in E. coli of natural folding-assisting molecules, termed chaperones, to improve production of the heterologous protein.

[0012] Genetic modification has proved successful with a number of proteins (Knappik, A. and Plückthun, A. (1995) Prot. Eng. 8:81-89; Wall, J. G. and Plückthun, A. (1999) Prot. Eng. 12:605-611) but remains severely limited by the fact that solutions to expression problems that result from mutagenic modification are likely to be highly specific for the particular protein being expressed—whereas solutions that could be applied to all heterologous proteins expressed in E. coli would eliminate the need for labour-intensive, highly time consuming mutagenic studies to be repeated for each protein being produced.

[0013] Manipulation of the growth environment, for example, by modifying nutrients and temperature, has been shown to have a mildly positive effect in a number of cases, but would ultimately be expected to improve expression of reasonably efficiently expressed proteins rather than being able to overcome serious difficulties of expression or folding of specific proteins. The approach that appears to offer most hope in terms of a generally applicable solution is that of co-expressing folding assisting molecules that will enable the host E. coli cells to correctly express any or all heterologous proteins, without the need for further optimisation. To date, no single molecule has been identified, however, that improves the production of all heterologous proteins studied and, thus, again the difficulty arises of having to individually optimise expression for each heterologous protein.

[0014] Thus, a generally applicable solution to expressing traditionally “difficult” proteins in E. coli would make a highly significant contribution to the field of heterologous protein production.

SUMMARY OF THE INVENTION

[0015] The invention provides a polypeptide factor derived from a thermophilic eubacterial species, said polypeptide factor having universal protein expression-assisting activity.

[0016] Preferably, the thermophilic eubacterial species is Thermus thermophilus.

[0017] Further, preferably, the polypeptide factor has an amino acid sequence defined as amino acid position 1 to amino acid position 291 in FIG. 2 (SEQ ID NO:1). This polypeptide factor has been named the CzrB protein as hereinafter described. Said polypeptide factor described and characterised herein has the potential to overcome, by a mechanism that is independent of the protein being expressed, many of the difficulties associated with expressing proteins, more particularly eucaryotic proteins in E. coli.

[0018] According to one embodiment of the invention, the CzrB polypeptide factor is the full length protein of 291 amino acids having SEQ ID NO:1 hereinbefore specified.

[0019] According to an alternative embodiment of the invention, the CzrB polypeptide factor is a truncated form of the CzrB protein, namely a polypeptide factor of 92 amino acids having SEQ ID NO:2.

[0020] The CzrB protein from T. thermophilus either in its full length or truncated forms described herein has the potential to act as a universal protein expression-assisting molecule which can increase the yields of all heterologous proteins produced in E. coli as hereinafter described.

[0021] It is expected that the truncated version of CzrB, containing a putative 92 amino acids as opposed to the 291 of the mature CzrB protein will lead to significantly higher improvements in protein yields upon over-expression from a better regulated promoter. The truncated protein is less than one-third the size of the mature protein and thus is likely to accumulate to much higher levels and at lower metabolic expense to the expressing cell. Furthermore, the truncated protein is also unlikely to be inserted into the cell membrane in the host bacterial cell and, thus, less likely to interfere with normal cell functioning if expressed at greatly elevated levels in the cell under the control of a strong promoter.

[0022] The invention also provides an isolated DNA sequence encoding each of the polypeptide factors hereinbefore defined.

[0023] These isolated DNA sequences include a DNA sequence having SEQ ID NO:3 encoding the polypeptide factor having SEQ ID NO:1 and a DNA sequence SEQ ID NO:4 encoding the polypeptide factor having SEQ ID NO:2.

[0024] The invention also provides an isolated DNA sequence comprising the structural gene encoding the polypeptide factor having SEQ ID NO: 1 and a flanking sequence containing a control element for the expression of said polypeptide factor. The flanking sequence can be either a downstream sequence or an upstream sequence or both. One such sequence is SEQ ID NO:5.

[0025] The invention also provides a method for increasing production of heterologous proteins in a bacterial host cell, which comprises contacting said bacterial host cell with an effective amount of the polypeptide factor as hereinbefore defined during the expression of said heterologous protein.

[0026] Preferably, the bacterial host cell is an E. coli host cell.

[0027] The invention also provides a vector comprising an isolated DNA sequence as hereinbefore defined.

[0028] The invention further provides a host cell containing an isolated DNA sequence as hereinbefore defined.

[0029] According to a further embodiment of the invention there is provided a method for increasing production of heterologous protein in a bacterial host cell, which method comprises cultivating said host cell under conditions permitting expression of a DNA sequence as hereinbefore defined.

[0030] Preferably, the heterologous protein is a eucaryotic protein.

[0031] According to a further embodiment of the invention there is provided a method for the production of heterologous functional protein in an E. coli host cell, said method comprising co-cultivating DNA for said heterologous functional protein with a DNA sequence as hereinbefore defined.

[0032] The invention also provides a method for the production of heterologous function protein in an E. coli host cell, which method comprises co-expression of a polypeptide factor as hereinbefore defined.

[0033] According to a further embodiment of the invention there is provided a method of reducing stress in an expressing bacterial cell, which method comprises co-expressing a heterologous protein and a polypeptide factor as hereinbefore defined.

[0034] According to a still further embodiment of the invention there is provided a method of optimising expression of a heterologous protein in an expressing bacterial cell, which method comprises co-expressing the heterologous protein and a polypeptide factor as hereinbefore defined.

[0035] The invention also provides an antibody to a polypeptide factor as herebefore defined.

[0036] The invention also provides a method of purifying a protein with universal protein expression-assisting activity, said method comprising contacting a cell extract with an antibody as hereinbefore defined.

[0037] The invention also provides a polypeptide factor as hereinbefore defined which has homology with metal ion efflux proteins from other eucaryotic species.

[0038] The invention also provides a polypeptide factor as hereinbefore defined which confers on E. coli increased resistance to heavy metal ions.

BRIEF DESCRIPTION OF FIGURES

[0039]FIG. 1 is an agarose gel depicting the results of screening a bacteriophage library as described in Example 1;

[0040]FIGS. 2A and 2B is an alignment of the T. thermophilus CzrB amino acid sequence with homologues from a number of species as described in Example 2;

[0041]FIG. 3 is a graph of intracellular zinc concentration (mg/g dry weight cells) for a number of E. coli clones as described in Example 3;

[0042]FIG. 4 is a graph of intracellular zinc concentration (mg/g dry weight cells) versus time (h)) following zinc efflux from E. coli cells containing T. thermophilus czrB as described in Example 3.

[0043]FIG. 5 is a graph of O.D. 600 versus time after induction (hours) depicting growth of E. coli clones containing T. thermophilus czrB as described in Example 4;

[0044]FIG. 6 depicts phage titers (cfu) of clones containing pHB102 phagemid vector or pHB102-czrB determined 10 and 22 h. after induction as described in Example 5; and

[0045]FIG. 7 depicts O.D. 405 resulting from anti-FITC ELISAs carried out on the clones of FIG. 6 determined 10 and 22 hr. after induction as described in Example 5.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0046] Protein folding is highly sensitive to increases in temperature and thus a thermophilic bacterial species namely, the thermophilic eubacterium T. thermophilus was screened for possible novel or additional folding-assisting factors that might enable it to carry out folding effectively at high temperatures. Many proteins from T. thermophilus have also been functionally produced in E. coli, indicating recognition of its control elements by the mesophile. Furthermore, even though a thermophile, many of the enzymes of T. thermophilus function at E. coli 's growth temperature, allowing for phenotypic screening of gene libraries as hereinafter described.

[0047] The screening was based on identifying T. thermophilus molecules that might improve the ability of host E. coli cells to express a known, poorly folding antibody molecule. To this end phage display technology was carried out, in which antibodies were displayed on the surface of bacteriophage particles. A phage display system is an in vitro approach that mimics the human immune system by generating a large, diverse collection of antibody molecules, expressed on the surface of bacteriophage (“phage”) particles, followed by selection of phage-antibody partners with desired binding specificities from the library. The displayed antibodies were first expressed in the host E. coli cells, leading to a requirement for correct folding for display. Bacteriophage particles were then subjected to affinity selection on an immobilised ligand, resulting in selection of antibody molecules that displayed an antibody of a particular binding specificity, depending on the identity of the immobilised ligand. While phage display technology is normally used to select antibodies from libraries of molecules of different binding specificities (McCafferty, J. et al (1990) Nature 348:552-554), as hereinafter exemplified in the same antibody was cloned onto all phage particles in accordance with the present invention. A genomic DNA library, generated from T. thermophilus, was also cloned into the expressing E. coli cells. Thus, the difference between E. coli clones in the resultant library was merely the identity of the T. thermophilus DNA contained within the cells, such that affinity selection of one bacteriophage particle over another would be determined by the Thermus gene and, in turn, its effect on the expression of the displayed antibody in the host E. coli cells.

[0048] Replication in E. coli gives rise to random mutations in the isolated antibodies with the result that affinity maturation (selection of higher affinity molecules) occured upon repeated cycles of antigen binding and re-infection. Critically, the production of phage particles in the host E. coli cells in this manner necessitates correct folding of not only the native phage proteins but also the displayed recombinant protein if it is to be effectively displayed and, thus, affinity selected.

[0049] Thus, a modified phage display system, in which all phage particles expressed the same, poorly-folding antibody molecule on their surface was utilised in accordance with the present invention. A chromosomal DNA library from T. thermophilus was also cloned into the phage vector containing the antibody fragment. As all phage particles contained the same antibody fragment—and should therefore display antibodies with identical affinities for the immobilised ligand—selection should be dependent on the efficiency of folding (and, thus, efficiency of display) of the molecule rather than the strength of the binding event. Thus, any T. thermophilus gene encoding a protein that facilitated expression of the recombinant antibody fragment would lead to improved production and display of the cloned antibody fragment and lead in turn to isolation of that clone from the phage antibody library (Spada, S. et al (2002) Extremophiles 6:301-8).

[0050] Screening of the T. thermophilus library led to the isolation of the czrB gene, both in a full-length and a truncated form. Sequence analysis indicated homology with metal ion efflux proteins from a number of species. The gene was re-cloned to eliminate flanking partial gene sequences in the isolated clones and the resultant clones were analysed in a number of ways. E. coli clones containing the czrB gene were found to exhibit increased resistance to cadmium and zinc, with zinc, cadmium and cobalt all found to induce the resistance to zinc ions. Measurement of intracellular zinc concentrations over time indicated that the CzrB protein was active in efflux of zinc from the E. coli cells as a mechanism of mediating this resistance. Furthermore, clones containing the czrB gene were shown to grow more rapidly and exhibited delayed cell lysis than clones lacking the gene, when this growth was associated with antibody and bacteriophage production. This led in turn to higher antibody yields in the former cells. Further analysis of this effect indicated that the CzrB protein did not appear to interact at the molecular level with either the antibody or the bacteriophage particles and could thus be considered a true, general “stress reliever” of the E. coli cells. For this reason it is envisaged that the polypeptide factors according to the invention will have widespread application in increasing production levels of heterologous proteins produced in E. coli in general, as stated above.

[0051] The T. thermophilus gene and gene product described herein have the potential to be developed into a universal cure for difficulties associated with the expression of eucaryotic proteins in E. coli. While other workers have identified molecules and techniques that assist the production of individual proteins in E. coli expression hosts, a critical aspect of the present invention is the observation that the effects on the expressing host—of improving growth, delaying cell lysis and, thus, increasing yields of heterologous protein produced—are independent of the identity of the protein being produced. Thus, coexpression of a DNA sequence according to the invention in currently available periplasmic expression vector systems is expected to confer such growth improvements on E. coli cells expressing any heterologous protein, irrespective of its source, with concomitant improvements in protein yields.

[0052] While the czrB gene was expressed under the control of its own, natural promoter as described in the following Examples, it will be clear to those skilled in the art that further improvements in E. coli cell growth and protein yields can be expected if czrB is expressed from a stronger, more standard promoter used in cloning and expression experiments, such as for example Plac. This would be expected to yield significantly higher CzrB levels in the expressing cell, which would be likely to exhibit increased benefits in terms of heterologous protein expression and yields.

[0053] The invention will be further illustrated by the following Example

EXAMPLE 1 Construction and Screening of the T. thermophilus Genomic Library

[0054] The T. thermophilus genomic library was constructed as follows (Spada S. et al (2001) DNA Seq 11:507-514.): a 5 ml culture of T. thermophilus KT8 was harvested at an OD₆₀₀ of 1.8 and the cell pellet was resuspended in 0.5 ml STE buffer (10 mM Tris, 100 mM NaCl, 1 mM EDTA, pH 8.0). RNase A was added to a final concentration of 100 μg/ml, SDS to 8.5 mg/ml and proteinase K to 100 μg/ml. Incubation for 2 h at 37° C. was followed by two phenol extractions, three phenol/chloroform/isoamyl alcohol extractions, ethanol precipitation and resuspension in 100 μl TE buffer. The T. thermophilus chromosomal DNA was partially digested using Sau3AI restriction enzyme in order to maximise the yield of DNA fragments in the 1-5 kb range. Fragments in this size range were purified using a QIAEXII agarose gel DNA extraction kit (Qiagen) and cloned into a BglII-digested pHB102 phagemid vector containing the poorly-folding anti- fluorescein-isothiocyanate (FITC) scFv antibody fragment (Bothmann, H. and Plückthun, A. (1998) Nat Biotechnol 16:376-380.). The library was transformed into E. coli XL1-Blue cells and the resultant library was estimated by NotI digestion to contain in the region of 1.1×10⁴ clones. As this was calculated to be in excess of the size required to contain all T. thermophilus genes, based on the size of the T. thermophilus genome, screening of the library was initiated.

[0055] Phage production was induced overnight in E. coli cells harbouring the Thermus library and five rounds of fluorescein-isothiocyanate (FITC), the antigen recognised by the displayed antibody fragment, were carried out as follows: E. coli cells harbouring the Thermus library were inoculated to an OD₆₀₀≦0.05 in 10 ml 2×YT medium containing tetracycline (15 μg/ml), additional salts (8.6 mM NaCl; 2.5 mM KCl; 10 mM MgCl₂) and 0.4% glucose. After 1 h at 37° C., 30 μg/ml chloramphenicol was added, followed by 10¹⁰ pfu of helper phage (VCSM13 helper phage; Stratagene) at an OD₆₀₀ of 0.5. Incubation at 50° C. for 5 min was followed by the addition of 50 ml 2×YT containing tetracycline, chloramphenicol, additional salts and 0.5 mM isopropyl-β-D-thiogalactoside (IPTG). The culture was shaken at 40° C. for 2 h and, after the addition of 30 μg/ml kanamycin, grown for a further 12-14 h at 40° C. Phage particles were precipitated from culture supernatants by two PEG precipitation steps and resuspended in 1 ml PBS (8 g NaCl, 0.24 g KH₂PO₄, 1.44 g Na₂HPO₄, 0.2 g KCl in 1 L, pH 7.4). Immunotubes (Nunc) were coated overnight at 4° C. with 1 μg/ml fluorescein-isothiocyanate coupled to bovine serum albumin (FITC-BSA) (Bothmann, H. and PlÜckthun, A (1998) supra) in PBS. Blocking was with 5% skimmed milk in PBST (PBS containing 0.05% Tween-20) for 2 h at 37° C., followed by dilution of 800 μl of the phage solution in 3.3 ml of 2% skimmed milk in PBST and incubation in the tubes for 2 h at 25° C. Twenty washes with PBST and two with PBS were followed by elution of bound particles for 10 min at room temperature using 1 ml 0.1 M glycine/HCl (pH 2.2). The eluate was neutralised immediately with 60 μl of 2 M Tris and used for reinfection of E. coli; this procedure was repeated for five rounds of phage selection on immobilised FITC and reinfection (“panning”). After each panning round DNA from the phage pool was digested with NotI to check for enrichment of Thermus DNA inserts. After the third round, a DNA fragment of approximately 1.2 kb began to appear in the digested library pool, as well as a less intense band of 1.8 kb which became considerably enriched by the fifth panning round as shown in FIG. 1.

[0056] In FIG. 1 the molecular weight marker (DNA Molecular Weight Marker XIV from Roche Applied Science) is in lane 1 and control, undigested phagemid DNA in lane 2. The result of restriction analysis of phage pools from panning rounds 3, 4 and 5 are shown above in lanes 3-5, respectively.

[0057] These results were a clear indication that selection of specific clones was occurring in the library. Therefore, after the fifth panning round, 30 individual clones were isolated and analysed by NotI digestion in order to determine the size of their cloned T. thermophilus gene. One clone had an insert of approximately 1.8 kb, 2 clones had inserts of 1.2 kb (which corresponded to the sizes observed in the library pool analysis), 25 clones had inserts of between 50 and 280 bp and the remaining 2 clones no insert. As approximately 90% of clones had been determined to contain inserts of 1-5 kb in analysis of the original library, the smaller inserts observed in 27 of 30 clones after panning was interpreted as evidence for a strong selective pressure against E. coli cells retaining large sections of DNA which provided no benefit to the cell. It was speculated, therefore, that the T. thermophilus genes retained (and selected) by E. coli clones under such conditions should confer a strong advantage upon the cells and the basis of that selective advantage was then determined in the 3 clones identified with the larger T. thermophilus DNA inserts.

EXAMPLE 2 Identification and Analysis of T. thermophilus czrB

[0058] Clones containing the larger fragments described in Example 1 were sequenced to identify the isolated Thermus genes. Sequencing of the isolated 1.8 kb clone revealed an insert of 1743 bp, containing a single complete open reading frame (ORF) of 876 bp. BLASTx analysis of the complete ORF using the EMBL database revealed homology to cation efflux system proteins, mostly termed Czr (for cadmium-zinc resistance) or CzcD (for cadmium-zinc-cobalt resistance), from a variety of organisms. Based on experimental analysis, the T. thermophilus gene was named czrB, after the Staphylococcus aureus gene (Kuroda, M. et al (1999) Microbiol Immunol 43:115-125). Multiple sequence alignments were generated with homologous proteins using CLUSTALw as depicted in FIG. 2.

[0059]FIG. 2 shows the alignment of the T. thermophilus CzrB amino acid sequence identified herein with homologues from Ralstonia eutropha (CzcD, accession number P13512), S. aureus (CzrB, Q9ZNF5), rat (Rattus norvegicus) (Znt1, Q62720) and Saccaromyces cerevisiae (Zrc1, P20107). Emboldening indicates residues identical to the T. thermophilus sequence and italics residues homologous to the Thermus sequence. The putative translation start site for the partial czrB gene isolated from the library (Met₂₀₀) is boxed.

[0060] Of the known homologues, only those from rat, S. cerevisiae, R. eutropha-like CH34 (previously Alcaligenes eutrophus CH34) and S. aureus have been phenotypically characterised, with the main structural difference between the proteins being the extended loops between putative transmembrane segments in eucaryotic species. PSORT II (Nakai, K. and Kanehisa, M. (1992) Genomics 14:897-911) was employed for subcellular localisation predictional analysis, which envisaged the Thermus protein as a cytoplasmic membrane protein of molecular mass 31233 Da, while a modified hidden Markov model was utilised for prediction of transmembrane helices (Krogh, A. et al (2001) J Mol Biol 305:567-580.) and indicated that it contained six putative membrane-spanning α-helices, of which the 4 N-terminal spanners were highly hydrophobic, features conserved in other CzcD-like proteins. The Thermus structural gene also had a % GC content and amino acid composition typical of genes from thermophilic species.

[0061] As the sequence of the T. thermophilus gene had not previously been reported, it was re-amplified from the T. thermophilus genomic DNA, cloned into pUC19 and re-sequenced in order to confirm the original sequence. As well as confirming the original sequence, this re-cloning served to eliminate partial structural genes on either side of the czrB gene in the original clone, in order to eliminate possible interfering materials in subsequent characterisation of the effects of CzrB. 100 bp was retained upstream and 78 bp downstream of the czrB gene in this re-cloning, however, as these regions were expected to contain any control elements from the T. thermophilus chromosome, thus allowing study of not only the CzrB protein, but also the control of its expression in vivo.

[0062] Sequencing of the two 1.2 kb clones isolated during library screening revealed identical partial copies of czrB, encoding the 108 C-terminal amino acids of the 291 residue protein. It was determined that the first consensus ATG in this truncated czrB gene occurred at Met₂₀₀ and was closely preceded by a putative ribosome binding site; therefore, it was concluded that translation most likely begins in these truncated genes at residue 200 and yields a 92 amino acid peptide that corresponds to the C-terminal, cytoplasmic tail of the mature CzrB molecule shown in FIG. 2. Subcellular localisation analysis of this putative polypeptide (Nakai, K. and Kanehisa, M. (1992) supra) predicted it to form a soluble, cytoplasmic molecule in the cell.

EXAMPLE 3 Heavy Metal Analysis

[0063] Given that the czrB gene and its truncated form that were isolated from the phage display screening described in Example 2 showed homology to cation efflux proteins, we investigated whether the isolated clones exhibited activities similar to those reported for homologous proteins in other species. Minimal inhibitory concentrations (MICs) for metal cations were therefore measured for cells with and without the cloned czrB gene in order to investigate whether the T. thermophilus protein protected host E. coli cells grown in high concentrations of heavy metals. E. coli clones were grown in LB medium containing 100 μg/ml ampicillin and 25 μg/ml streptomycin for 90 min at 37° C. This was carried out with and without addition of 165 μM ZnCl₂, 220 μM CoCl₂ or 80 μM CdCl₂ (chosen as approximately 10% of MICs). Following dilution in LB, 10³-10⁴ cells were spread on LB agar (plus ampicillin and streptomycin) containing ZnCl₂ (at concentrations ranging from 1.4 mM to 2.9 mM at 0.1 mM intervals), CoCl₂ (1.7 mM to 2.2 mM with 0.1 mM steps) or CdCl₂ (from 0.6 mM to 1.2 mM with 0.1 mM steps). Growth of E. coli was measured after 24 h and 40 h, with MICs of the three metals defined as the lowest concentrations not allowing detectable E. coli growth after 40 h at 37° C. MIC determinations were carried out three times with each clone and metal ion. E. coli cells containing the T. thermophilus czrB gene exhibited a significantly higher MIC for Zn²⁺ than cells containing pUC alone, while the cadmium MIC increased only slightly and cobalt resistance was unaffected by the presence of czrB as shown in Table 1. TABLE 1 Minimal inhibitory concentrations (MICs) of ZnCl₂, CoCl₂ and CdCl₂ determined for E. coli JM83, JM83 containing pUC and JM83 containing pUC with cloned czrB. MIC (mM) Clone pre-induction Zinc Cobalt Cadmium JM83 none 2.0 2.1 0.9 +pUC none 1.6 2.1 0.9 +pUC zinc 1.6 2.1 0.9 +pUC cobalt 1.6 2.1 0.9 +pUC cadmium 1.6 2.1 0.7 +pUC-czrB none 1.9 2.1 1.0 +pUC-czrB zinc 2.5 2.1 1.0 +pUC-czrB cobalt 2.1 2.1 1.1 +pUC-czrB cadmium 2.2 2.1 0.9

[0064] Induction of metal resistance in E. coli cells carrying czrB was found to have a significant effect on metal tolerance of cells over the subsequent growth period, but with no difference observed between relative MICs measured after 24 or 40 h. Resistance to zinc mediated by the Thermus czrB could be induced by pre-incubation of E. coli cells with zinc, cadmium or cobalt as shown in Table 1. Thus, curiously, the Thermus protein appears to recognise cobalt and yet not to transport the cation. The ability to induce zinc tolerance was considerable, with the zinc MIC increasing from 1.6 mM (no czrB) to 1.9 mM (with czrB) to 2.5 mM (with czrB pre-induced by zinc) in the E. coli cells. The ability to induce cadmium resistance was poor, however, with only slight, non-statistically significant, increases in MIC inducible with cadmium or cobalt, and no detectable effect with zinc. No increase in zinc MICs was seen upon pre-incubation of wild-type E. coli cells with zinc.

[0065] Metal resistances of the type observed with the czrB clones can be the result of metal sequestering or modification of metal transport processes. However, based on reports of CzrB homologues in other organisms it was postulated that the protein CzrB provided resistance to heavy metal ions by an efflux mechanism rather than by metal sequestering. Therefore, the ability of the T. thermophilus CzrB protein to pump zinc ions out of the E. coli cells in which it was expressed was investigated. Intracellular zinc concentrations were measured as follows: E. coli clones were grown at 37° C. in LB medium (100 μg/ml ampicillin, 25 μg/ml streptomycin) in the presence or absence of 165 μM ZnCl₂ until an OD₆₀₀ of 1.0 was reached. After addition of 1, 2 or 5 mM ZnCl₂, growth was continued for 30 min, 1 h or 2 h, with control cultures grown in the absence of ZnCl₂. Samples (20 ml) of each culture were centrifuged at 8000 g for 20 min at 4° C. and cell pellets were washed in 4 ml LB medium and in 4 ml 0.1 N HNO₃. Following 15 min at 121° C., pellets were dissolved in 500 μl of H₂SO₄ and approximately 150 μl of HNO₃ was added dropwise until the solution went clear. Six ml of water was added, followed by centrifugation at 8000 g for 25 min. The zinc concentration was measured in the supernatant using an atomic absorption spectrophotometer (Varian SpectrAA-400 Plus), with standard solutions prepared immediately before use from commercial standards (Fisher Scientific). A calibration curve relating OD₆₀₀ to cell dry weight was used to calculate intracellular zinc concentrations at time of harvesting. The results are shown in FIG. 3 which depicts a quantification of intracellular zinc levels in E. coli clones: JM83 (“JM83”), JM83 containing pUC (“pUC”), and JM83 containing pUC-czrB without (“czrB”) and with (“czrB (Zn)”) a zinc pre-induction step. Results of 1 h incubations in 0, 1, 2 and 5 mM extracellular zinc concentrations are shown.

[0066] As indicated in FIG. 3, the presence of czrB was found to significantly reduce the levels of zinc in E. coli cells, whereas pUC alone led to elevated intracellular zinc levels. These results indicated that increased metal resistance was mediated by modification of either influx or efflux activity. This effect was particularly evident at high extracellular zinc concentrations that led to elevated initial intracellular concentrations in E. coli cells. Pre-exposure of cells to zinc further reduced cellular levels in czrB clones in high zinc environments, indicating an inducible resistance mechanism, as observed in MIC experiments.

[0067] A time course experiment was carried out to distinguish between reduced influx and increased efflux as the cause of the reduced cellular accumulation of zinc. The results are shown in FIG. 4 which depicts the results of an analysis of zinc efflux from E. coli cells containing T. thermophilus czrB. Clones shown were grown in 0 mM (triangles), 1 mM (circles), 2 mM (squares) or 5 mM (diamonds) ZnCl₂. Empty symbols show the same clones subjected to pre-exposure to 165 μM ZnCl₂ prior to analysis. Intracellular zinc concentrations decreased significantly over the analysis period, with the rate of ion removal increased upon pre-exposure of cells to zinc, indicating that the protective mechanism of czrB involves an inducible process of efflux of metals from the cell.

EXAMPLE 4 Investigation of Effects on Heterologous Protein Production

[0068] The strong selection of clones that had eliminated all or part of their Thermus insert during phage panning (27 of 30 clones analysed were found to have inserts of <300 bp) indicated that czrB exerted a strong positive effect on its host cells merely to be retained throughout library screening. In addition, the czrB gene was contained in all 3 large-insert clones selected from the library, in one case as a full-length molecule and in the other two as identical partial sequences. The observed effects of czrB on metal resistance of host E. coli cells were of no apparent advantage (or relevance) during the phage display procedure. An investigation of why czrB, in both its full-length and partial forms, should be so strongly selected in the library screening experiments was then investigated. Thus, E. coli clones were analysed under recombinant antibody and phage production conditions in order to investigate the basis of the effect of CzrB. These experiments were designed to determine if (i) czrB led to increased phage titres in host E. coli cells; (ii) czrB led to improved folding of the recombinant anti-FITC antibody, co-expressed in the host E. coli cell; and (iii) czrB had any effect on the growth of E. coli cells expressing the anti-FITC antibody.

[0069]E. coli cells infected with phage were grown as described in Example 1. The OD₆₀₀ was read at hourly intervals for the first 8-10 h after induction. The results are shown in FIG. 5 which depicts growth of E. coli clones containing T. thermophilus czrB. Growth characteristics of E. coli clones containing pHB102 phagemid vector alone (circles) or with czrB (triangles), a partial czrB gene (squares) or random T. thermophilus DNA (diamonds) as an insert are shown. The experiment was carried out during bacteriophage and recombinant antibody production in the E. coli cells. This experiment revealed that czrB-containing clones grew considerably better than cells containing just the phagemid vector with delayed cell lysis and greater than 2-fold higher cell densities attained. These cell density differences were maintained after 22 h of induction (data not shown) and this improved growth of czrB-containing clones is believed to account for selection of the gene from the library. Isolated clones containing the partial czrB insert exhibited growth characteristics intermediate between the full-length czrB clone and cells containing the phagemid vector, while a randomly selected control clone with a 2 kb insert displayed significantly poorer growth than clones containing the vector alone. This result also provided an insight into how the original library became biased in favour of clones containing small sized or no Thermus inserts in the absence of a phenotypic benefit associated with the cloned DNA.

EXAMPLE 5 Determination of Phage Titers and the Amount of Antibody Displayed in Funcational Form on the Phage Surface

[0070] Culture samples from the growth experiment described in Example 4 were also collected to determine both phage titers and the amount of antibody displayed in functional form on the phage surface. Individual clones were analysed by restriction digestion after 22 h of induction to confirm that they contained both the Thermus insert and the recombinant antibody gene. Clones were analysed in at least three independent experiments and while absolute OD values, phage titers and ELISA readings varied between experiments, the respective patterns of growth and production exhibited by individual clones remained highly consistent throughout. ELISA analysis was used to determine the effects of co-expressing the czrB gene on the functionality of the phage-displayed antibody protein. Immunoplate wells (Nunc) were coated with 100 μl of FITC-BSA and blocked with 5% skimmed milk in PBST. After washing, 100 μl of phage solutions containing 0.5% skimmed milk were added and incubated for 2 h at 25° C. Phage particles were detected using a peroxidase-conjugated anti-M13 antibody (1:3000 in PBST; Amersham Pharmacia Biotech Inc.) and development was carried out using a BM Blue POD soluble substrate (Roche Diagnostics). After stopping the reaction using 25 μl of 1 N H₂SO₄, the absorbance was read at 405 nm. E. coli cultures containing czrB were found to exhibit two-fold higher phage titres and to produce more than twice the amount of functional antibody as cultures containing the vector alone. The results are depicted in FIGS. 6 and 7.

[0071]FIG. 6 depicts phage titers of clones expressing czrB. The phage titers of clones containing pHB102 phagemid vector or pHB102-czrB expressed as colony forming units (cfu) were determined 10 and 22 h after induction.

[0072]FIG. 7 depicts ELISA analysis of clones expressing czrB. Anti-FITC ELISAs were carried out on the same clones as in the case of FIG. 6 so as to investigate the functionality of bacteriophage-displayed FITC-binding antibody fragments. Results are shown for samples taken 10 and 22 h after induction.

[0073] Cells containing the partial czrB gene showed signals intermediate between the full-length gene and the control culture in both phage titre and ELISA studies (data not shown). This led to the conclusion that, while the transmembrane domains of the T. thermophilus protein are required for its full effect to be achieved, the putative cytoplasmic tail of CzrB alone makes a significant contribution to the beneficial effects of the protein observed in E. coli. While the truncated CzrB construct according to the invention was not tested in efflux experiments, other workers have found that the C-terminal 62 and 72 amino acids from rat ZnT-1 and R. eutropha CzcD, respectively, are inessential for the protein's role in cation efflux, suggesting that the roles of T. thermophilus CzrB in metal efflux and in facilitating E. coli growth under recombinant protein production conditions occur via distinct mechanisms. Finally, the increased yields observed with both the full length and truncated versions of czrB were proportional to, and therefore apparently directly attributable to, the increased cell densities observed in E. coli cultures containing czrB, indicating that the czrB gene product appeared to have no direct effect on cellular antibody expression or bacteriophage production. Rather, its effect appeared to reside in relieving the physiological stresses typically associated with recombinant protein production in E. coli, thus allowing improved growth, higher culture densities and increased yields of recombinant protein in the cultures.

[0074] To investigate this theory further, czrB was expressed from a lac promoter in a standard pUC-based expression vector, in the absence of bacteriophage particles and the recombinant antibody used in experiments thus far. The same pattern of E. coli cell growth, relative to cells containing the vector alone, was observed upon Plac induction (data not shown), confirming that the effect of CzrB appears to be to improve E. coli physiology, at least in the presence of pUC-based vectors, rather than to interact directly with the antibody or bacteriophage molecules in the cell. This raises the possibility that CzrB, in its full length form or as a truncated version, might function as an “universal chaperone”, which would facilitate the expression of any recombinant or heterologous proteins produced in an E. coli host.

1 9 1 291 PRT Thermus thermophilus CzrB 1 Met Ala Glu Gly Ala Ala Arg Leu Ser Leu Val Val Ala Leu Leu Val 1 5 10 15 Leu Gly Leu Lys Ala Phe Ala Tyr Leu Leu Thr Gly Ser Val Ala Leu 20 25 30 Leu Ser Asp Ala Leu Glu Ser Leu Val Asn Val Ala Ala Ala Leu Ala 35 40 45 Ala Leu Leu Ala Leu Arg Val Ala Arg Lys Pro Pro Asp Gln Asn His 50 55 60 Pro Phe Gly His Thr Lys Ala Glu Tyr Val Ser Ala Val Leu Glu Gly 65 70 75 80 Val Leu Val Val Leu Ala Ala Leu Trp Ile Ala Arg Glu Ala Leu Pro 85 90 95 Arg Leu Leu His Pro Val Pro Leu Glu Gly Leu Gly Leu Gly Leu Gly 100 105 110 Val Ser Leu Leu Ala Ser Leu Leu Asn Gly Leu Leu Ala Tyr His Leu 115 120 125 Leu Lys Glu Gly Arg Arg His Arg Ser Pro Ala Leu Thr Ala Asp Gly 130 135 140 Tyr His Val Leu Ser Asp Val Leu Thr Ser Leu Gly Val Val Leu Gly 145 150 155 160 Val Gly Leu Ala Gly Leu Thr Gly Leu Trp Val Leu Asp Pro Leu Leu 165 170 175 Ala Leu Ala Val Ala Gly Gln Ile Leu Phe Leu Gly Tyr Arg Ile Val 180 185 190 Arg Glu Ser Val Gly Gly Leu Met Asp Glu Gly Leu Pro Pro Glu Glu 195 200 205 Val Glu Arg Ile Arg Ala Phe Leu Gln Glu Arg Ile Arg Gly Arg Ala 210 215 220 Leu Glu Val His Asp Leu Lys Thr Arg Arg Ala Gly Pro Arg Ser Phe 225 230 235 240 Leu Glu Phe His Leu Val Val Arg Gly Asp Thr Pro Val Glu Glu Ala 245 250 255 His Arg Leu Cys Asp Glu Leu Glu Arg Ala Leu Ala Gln Ala Phe Pro 260 265 270 Gly Leu Gln Ala Thr Ile His Val Glu Pro Glu Gly Glu Arg Lys Arg 275 280 285 Thr Asn Pro 290 2 92 PRT Thermus thermophilus CzrB 2 Met Asp Glu Gly Leu Pro Pro Glu Glu Val Glu Arg Ile Arg Ala Phe 1 5 10 15 Leu Gln Glu Arg Ile Arg Gly Arg Ala Leu Glu Val His Asp Leu Lys 20 25 30 Thr Arg Arg Ala Gly Pro Arg Ser Phe Leu Glu Phe His Leu Val Val 35 40 45 Arg Gly Asp Thr Pro Val Glu Glu Ala His Arg Leu Cys Asp Glu Leu 50 55 60 Glu Arg Ala Leu Ala Gln Ala Phe Pro Gly Leu Gln Ala Thr Ile His 65 70 75 80 Val Glu Pro Glu Gly Glu Arg Lys Arg Thr Asn Pro 85 90 3 873 DNA Thermus thermophilus CzrB 3 atggccgaag gcgccgcccg gttgagcctc gtcgtcgccc tcctcgtctt ggggctcaag 60 gccttcgcct accttctcac gggctcggtg gccctgctct cggacgccct cgagtccctg 120 gtgaacgtgg ccgcggccct cgccgccctc ctcgccctcc gggtcgcccg caagccgccg 180 gaccagaacc accccttcgg ccacaccaag gccgagtacg tttccgccgt cctggaaggg 240 gtgctggtgg tcttggccgc cctctggatc gccagggagg ccctgccccg cctcctccac 300 cccgtgcccc tcgagggctt gggcttgggg cttggggtga gcctcctcgc ctccctcctc 360 aacggcctcc tggcctacca cctcctgaag gagggccgcc gccaccgctc ccccgccctc 420 accgccgacg ggtaccacgt cctctccgac gtcctcacct ccttaggggt ggtcctgggc 480 gtgggcctcg ccgggctcac gggcctttgg gtcttggacc ccctcctcgc cctcgcggtg 540 gcgggccaga tcctcttcct gggctaccgc atcgtgcggg agtccgtggg agggcttatg 600 gacgagggcc tccctccgga ggaggtggag cgcatccgcg ccttccttca ggagcgcatc 660 cggggccggg ccctcgaggt ccacgacctc aagacgcgaa gggccggccc caggagcttc 720 ctggagttcc acctcgtggt gcggggggac acccccgtgg aggaggccca ccgcctctgc 780 gacgagttgg aaagggccct ggcccaggcc tttcccggcc ttcaggccac catccacgtg 840 gagcccgagg gcgagcggaa gcggacaaac ccc 873 4 276 DNA Thermus thermophilus CzrB 4 atggacgagg gcctccctcc ggaggaggtg gagcgcatcc gcgccttcct tcaggagcgc 60 atccggggcc gggccctcga ggtccacgac ctcaagacgc gaagggccgg ccccaggagc 120 ttcctggagt tccacctcgt ggtgcggggg gacacccccg tggaggaggc ccaccgcctc 180 tgcgacgagt tggaaagggc cctggcccag gcctttcccg gccttcaggc caccatccac 240 gtggagcccg agggcgagcg gaagcggaca aacccc 276 5 1020 DNA Thermus thermophilus CzrB 5 cctcaagccc aagaaggagg cggtggaaga aggggtctaa atggccgaag gcgccgcccg 60 gttgagcctc gtcgtcgccc tcctcgtctt ggggctcaag gccttcgcct accttctcac 120 gggctcggtg gccctgctct cggacgccct cgagtccctg gtgaacgtgg ccgcggccct 180 cgccgccctc ctcgccctcc gggtcgcccg caagccgccg gaccagaacc accccttcgg 240 ccacaccaag gccgagtacg tttccgccgt cctggaaggg gtgctggtgg tcttggccgc 300 cctctggatc gccagggagg ccctgccccg cctcctccac cccgtgcccc tcgagggctt 360 gggcttgggg cttggggtga gcctcctcgc ctccctcctc aacggcctcc tggcctacca 420 cctcctgaag gagggccgcc gccaccgctc ccccgccctc accgccgacg ggtaccacgt 480 cctctccgac gtcctcacct ccttaggggt ggtcctgggc gtgggcctcg ccgggctcac 540 gggcctttgg gtcttggacc ccctcctcgc cctcgcggtg gcgggccaga tcctcttcct 600 gggctaccgc atcgtgcggg agtccgtggg agggcttatg gacgagggcc tccctccgga 660 ggaggtggag cgcatccgcg ccttccttca ggagcgcatc cggggccggg ccctcgaggt 720 ccacgacctc aagacgcgaa gggccggccc caggagcttc ctggagttcc acctcgtggt 780 gcggggggac acccccgtgg aggaggccca ccgcctctgc gacgagttgg aaagggccct 840 ggcccaggcc tttcccggcc ttcaggccac catccacgtg gagcccgagg gcgagcggaa 900 gcggacaaac ccctgacgct cttttcctgc ccggcaaaaa agcgtaaact atggggcaaa 960 ggaggccccc atgcgcagga agcacgactg gctcagggaa acctatagga agagcctgga 1020 6 316 PRT Ralstonia eutropha CzcD 6 Met Gly Ala Gly His Ser His Asp His Pro Gly Gly Asn Glu Arg Ser 1 5 10 15 Leu Lys Ile Ala Leu Ala Leu Thr Gly Thr Phe Leu Ile Ala Glu Val 20 25 30 Val Gly Gly Val Met Thr Lys Ser Leu Ala Leu Ile Ser Asp Ala Ala 35 40 45 His Met Leu Thr Asp Thr Val Ala Leu Ala Ile Ala Leu Ala Ala Ile 50 55 60 Ala Ile Ala Lys Arg Pro Ala Asp Lys Lys Arg Thr Phe Gly Tyr Tyr 65 70 75 80 Arg Phe Glu Ile Leu Ala Ala Ala Phe Asn Ala Leu Leu Leu Phe Gly 85 90 95 Val Ala Ile Tyr Ile Leu Tyr Glu Ala Tyr Leu Arg Leu Lys Ser Pro 100 105 110 Pro Gln Ile Glu Ser Thr Gly Met Phe Val Val Ala Val Leu Gly Leu 115 120 125 Ile Ile Asn Leu Ile Ser Met Arg Met Leu Ser Ser Gly Gln Ser Ser 130 135 140 Ser Leu Asn Val Lys Gly Ala Tyr Leu Glu Val Trp Ser Asp Leu Leu 145 150 155 160 Gly Ser Val Gly Val Ile Ala Gly Ala Ile Ile Ile Arg Phe Thr Gly 165 170 175 Trp Ala Trp Val Asp Ser Ala Ile Ala Val Leu Ile Gly Leu Trp Val 180 185 190 Leu Pro Arg Thr Trp Ile Leu Leu Lys Ser Ser Leu Asn Val Leu Leu 195 200 205 Glu Gly Val Pro Asp Asp Val Asp Leu Ala Glu Val Glu Lys Gln Ile 210 215 220 Leu Ala Thr Pro Gly Val Lys Ser Phe His Asp Leu His Ile Trp Ala 225 230 235 240 Leu Thr Ser Gly Lys Ala Ser Leu Thr Val His Val Val Asn Asp Thr 245 250 255 Ala Val Asn Pro Glu Met Glu Val Leu Pro Glu Leu Lys Gln Met Leu 260 265 270 Ala Asp Lys Phe Asp Ile Thr His Val Thr Ile Gln Phe Glu Leu Ala 275 280 285 Pro Cys Glu Gln Ala Asp Ala Ala Gln His Phe Asn Ala Ser Pro Ala 290 295 300 Leu Val Gly Ser Lys Ser Leu Ala Ala Gly Gly Asn 305 310 315 7 325 PRT Staphylococcus aureus CzrB 7 Met Ser His Ser His His His Asp His Met His Ser His Val Thr Thr 1 5 10 15 Asp Asn Lys Lys Val Leu Phe Ile Ser Phe Leu Ile Ile Gly Leu Tyr 20 25 30 Met Phe Ile Glu Ile Ile Gly Gly Leu Leu Ala Asn Ser Leu Ala Leu 35 40 45 Leu Ser Asp Gly Ile His Met Phe Ser Asp Thr Phe Ser Leu Gly Val 50 55 60 Ala Leu Val Ala Phe Ile Tyr Ala Glu Lys Asn Ala Thr Thr Thr Lys 65 70 75 80 Thr Phe Gly Tyr Lys Arg Phe Glu Val Leu Ala Ala Leu Phe Asn Gly 85 90 95 Val Thr Leu Phe Val Ile Ser Ile Leu Ile Val Phe Glu Ala Ile Lys 100 105 110 Arg Phe Phe Val Pro Ser Glu Val Gln Ser Lys Glu Met Leu Ile Ile 115 120 125 Ser Ile Ile Gly Leu Ile Val Asn Ile Val Val Ala Phe Phe Met Phe 130 135 140 Lys Gly Gly Asp Thr Ser His Asn Leu Asn Met Arg Gly Ala Phe Leu 145 150 155 160 His Val Ile Gly Asp Leu Leu Gly Ser Val Gly Ala Ile Thr Ala Ala 165 170 175 Ile Leu Ile Trp Ala Phe Gly Trp Thr Ile Ala Asp Pro Ile Ala Ser 180 185 190 Ile Leu Val Ser Val Ile Ile Leu Lys Ser Ala Trp Gly Ile Thr Lys 195 200 205 Ser Ser Ile Asn Ile Leu Met Glu Gly Thr Pro Ser Asp Val Asp Ile 210 215 220 Asp Glu Val Ile Thr Thr Ile Lys Lys Asp Ser Arg Ile Gln Ser Val 225 230 235 240 His Asp Cys His Val Trp Thr Ile Ser Asn Asp Met Asn Ala Leu Ser 245 250 255 Cys His Val Val Val Asp His Thr Leu Thr Met Lys Glu Cys Glu Leu 260 265 270 Leu Leu Glu Asn Ile Glu His Asp Leu Leu His Leu Asn Ile His His 275 280 285 Met Thr Ile Gln Leu Glu Thr Pro Asn His Lys His Asp Glu Ser Ile 290 295 300 Ile Cys Ser Gly Thr His Ser His Ser His Asn His His Ala His His 305 310 315 320 His Ala His Val His 325 8 507 PRT Rattus norvegicus Znt1 8 Met Gly Cys Trp Gly Arg Asn Arg Gly Arg Leu Leu Cys Met Leu Leu 1 5 10 15 Leu Thr Phe Met Phe Met Val Leu Glu Val Val Val Ser Arg Val Thr 20 25 30 Ala Ser Leu Ala Met Leu Ser Asp Ser Phe His Met Leu Ser Asp Val 35 40 45 Leu Ala Leu Val Val Ala Leu Val Ala Glu Arg Phe Ala Arg Arg Thr 50 55 60 His Ala Thr Gln Lys Asn Thr Phe Gly Trp Ile Arg Ala Glu Val Met 65 70 75 80 Gly Ala Leu Val Asn Ala Ile Phe Leu Thr Gly Leu Cys Phe Ala Ile 85 90 95 Leu Leu Glu Ala Val Glu Arg Phe Ile Glu Pro His Glu Met Gln Gln 100 105 110 Pro Leu Val Val Leu Ser Val Gly Val Ala Gly Leu Leu Val Asn Val 115 120 125 Leu Gly Leu Cys Leu Phe His His His Ser Gly Glu Gly Gln Gly Ala 130 135 140 Gly His Gly His Ser His Gly His Gly His Gly His Leu Ala Lys Gly 145 150 155 160 Ala Arg Lys Ala Gly Arg Ala Gly Gly Glu Ala Gly Ala Pro Pro Gly 165 170 175 Arg Ala Pro Asp Gln Glu Pro Asp Gln Glu Glu Thr Asn Thr Leu Val 180 185 190 Ala Asn Thr Ser Asn Ser Asn Gly Leu Lys Ala Asp Gln Ala Glu Pro 195 200 205 Glu Lys Leu Arg Ser Asp Asp Pro Val Asp Val Gln Val Asn Gly Asn 210 215 220 Leu Ile Gln Glu Ser Asp Ser Leu Glu Ser Glu Asp Asn Arg Ala Gly 225 230 235 240 Gln Leu Asn Met Arg Gly Val Phe Leu His Val Leu Gly Asp Ala Leu 245 250 255 Gly Ser Val Ile Val Val Val Asn Ala Leu Val Phe Tyr Phe Ser Trp 260 265 270 Lys Gly Cys Thr Glu Asp Asp Phe Cys Val Asn Pro Cys Phe Pro Asp 275 280 285 Pro Cys Lys Ser Ser Val Glu Leu Met Asn Ser Thr Gln Ala Pro Met 290 295 300 His Glu Ala Gly Pro Cys Trp Val Leu Tyr Leu Asp Pro Thr Leu Cys 305 310 315 320 Ile Ile Met Val Cys Ile Leu Leu Tyr Thr Thr Tyr Pro Leu Leu Lys 325 330 335 Glu Ser Ala Leu Ile Leu Leu Gln Thr Val Pro Lys Gln Ile Asp Ile 340 345 350 Lys His Leu Val Lys Glu Leu Arg Asp Val Glu Gly Val Glu Glu Val 355 360 365 His Glu Leu His Val Trp Gln Leu Ala Gly Ser Arg Ile Ile Ala Thr 370 375 380 Ala His Ile Lys Cys Glu Asp Pro Ala Ser Tyr Met Gln Val Ala Lys 385 390 395 400 Thr Ile Lys Asp Val Phe His Asn His Gly Ile His Ala Thr Thr Ile 405 410 415 Gln Pro Glu Phe Ala Ser Val Gly Ser Lys Ser Ser Val Val Pro Cys 420 425 430 Glu Leu Ala Cys Arg Thr Gln Cys Ala Leu Lys Gln Cys Cys Gly Thr 435 440 445 Arg Pro Gln Val His Ser Gly Lys Glu Ala Glu Lys Ala Pro Thr Val 450 455 460 Ser Ile Ser Cys Leu Glu Leu Ser Glu Asn Leu Glu Lys Lys Pro Arg 465 470 475 480 Arg Thr Lys Ala Glu Gly Ser Val Pro Ala Val Val Ile Glu Ile Lys 485 490 495 Asn Val Pro Asn Lys Gln Pro Glu Ser Ser Leu 500 505 9 442 PRT Saccharomyces cerevisiae Zrc1 9 Met Ile Thr Gly Lys Glu Leu Arg Ile Ile Ser Leu Leu Thr Leu Asp 1 5 10 15 Thr Val Phe Phe Leu Leu Glu Ile Thr Ile Gly Tyr Met Ser His Ser 20 25 30 Leu Ala Leu Ile Ala Asp Ser Phe His Met Leu Asn Asp Ile Ile Ser 35 40 45 Leu Leu Val Ala Leu Trp Ala Val Asp Val Ala Lys Asn Arg Gly Pro 50 55 60 Asp Ala Lys Tyr Thr Tyr Gly Trp Lys Arg Ala Glu Ile Leu Gly Ala 65 70 75 80 Leu Ile Asn Ala Val Phe Leu Ile Ala Leu Cys Phe Ser Ile Met Ile 85 90 95 Glu Ala Leu Gln Arg Leu Ile Glu Pro Gln Glu Ile Gln Asn Pro Arg 100 105 110 Leu Val Leu Tyr Val Gly Val Ala Gly Leu Ile Ser Asn Val Val Gly 115 120 125 Leu Phe Leu Phe His Asp His Gly Ser Asp Ser Leu His Ser His Ser 130 135 140 His Gly Ser Val Glu Ser Gly Asn Asn Asp Leu Asp Ile Glu Ser Asn 145 150 155 160 Ala Thr His Ser His Ser His Ala Ser Leu Pro Asn Asp Asn Leu Ala 165 170 175 Ile Asp Glu Asp Ala Ile Ser Ser Pro Gly Pro Ser Gly Gln Ile Gly 180 185 190 Glu Val Leu Pro Gln Ser Val Val Asn Arg Leu Ser Asn Glu Ser Gln 195 200 205 Pro Leu Leu Asn His Asp Asp His Asp His Ser His Glu Ser Lys Lys 210 215 220 Pro Gly His Arg Ser Leu Asn Met His Gly Val Phe Leu His Val Leu 225 230 235 240 Gly Asp Ala Leu Gly Asn Ile Gly Val Ile Ala Ala Ala Leu Phe Ile 245 250 255 Trp Lys Thr Glu Tyr Ser Trp Arg Tyr Tyr Ser Asp Pro Ile Val Ser 260 265 270 Leu Ile Ile Thr Ile Ile Ile Phe Ser Ser Ala Leu Pro Leu Ser Arg 275 280 285 Arg Ala Ser Arg Ile Leu Leu Gln Ala Thr Pro Ser Thr Ile Ser Ala 290 295 300 Asp Gln Ile Gln Arg Glu Ile Leu Ala Val Pro Gly Val Ile Ala Val 305 310 315 320 His Asp Phe His Val Trp Asn Leu Thr Glu Ser Ile Tyr Ile Ala Ser 325 330 335 Ile His Val Gln Ile Asp Cys Ala Pro Asp Lys Phe Met Ser Ser Ala 340 345 350 Lys Leu Ile Arg Lys Ile Phe His Gln His Gly Ile His Ser Ala Thr 355 360 365 Val Gln Pro Glu Phe Val Ser Gly Asp Val Asn Glu Asp Ile Arg Arg 370 375 380 Arg Phe Ser Ile Ile Ala Gly Gly Ser Pro Ser Ser Ser Gln Glu Ala 385 390 395 400 Phe Asp Ser His Gly Asn Thr Glu His Gly Arg Lys Lys Arg Ser Pro 405 410 415 Thr Ala Tyr Gly Ala Thr Thr Ala Ser Ser Asn Cys Ile Val Asp Asp 420 425 430 Ala Val Asn Cys Asn Thr Ser Asn Cys Leu 435 440 

1. A polypeptide factor derived from a thermophilic eubacterial species, said polypeptide factor having universal protein expression-assisting activity.
 2. The polypeptide of claim 1, wherein the thermophilic eubacterial species is Thermus thermophilus.
 3. The polypeptide factor of claim 1 or 2, having an amino acid sequence defined as amino acid position 1 to amino acid position 291 in FIG. 2 (SEQ ID NO:1).
 4. The polypeptide factor of claim 3, having SEQ ID NO:2.
 5. An isolated DNA sequence encoding the polypeptide factor of claim
 1. 6. An isolated DNA sequence encoding the polypeptide factor of claim
 2. 7. An isolated DNA sequence encoding the polypeptide factor of claim 3 and having SEQ ID NO:3.
 8. An isolated DNA sequence encoding the polypeptide factor of claim 4 and having SEQ ID NO:4.
 9. An isolated DNA sequence comprising the structural gene encoding the polypeptide factor of claim 3 and a flanking sequence containing a control element for the expression of said polypeptide factor.
 10. An isolated DNA sequence according to claim 9 having SEQ ID NO:5.
 11. A method for increasing production of heterologous proteins in a bacterial host cell, which comprises contacting said bacterial host cell with an effective amount of the polypeptide factor of claim 1 during the expression of said heterologous protein.
 12. A method according to claim 11, wherein the bacterial host cell is an E. coli host cell.
 13. A vector comprising the isolated DNA sequence of claim
 5. 14. A host cell containing the isolated DNA sequence of claim
 5. 15. A method for increasing production of heterologous protein in a bacterial host cell, which method comprises cultivating said host cell under conditions permitting expression of said DNA sequence of claim 5 or
 9. 16. A method according to claim 15, wherein the heterologous protein is a eucaryotic protein.
 17. A method for the production of heterologous functional protein in an E. coli host cell, said method comprising co-cultivating DNA for said heterologous functional protein with the DNA sequence of claim 5 or
 9. 18. A method for the production of heterologous function protein in an E. coli host cell, which method comprises co-expression of said polypeptide factor of claim
 1. 19. A method of reducing stress in an expressing bacterial cell, which method comprises co-expressing a heterologous protein and the polypeptide factor of claim
 1. 20. A method of optimising expression of a heterologous protein in an expressing bacterial cell, which method comprises co-expressing the heterologous protein and the polypeptide factor of claim
 1. 21. An antibody to a polypeptide factor as defined in claim
 1. 22. A method of purifying a protein with universal protein expression-assisting activity, said method comprising contacting a cell extract with an antibody of claim
 21. 23. The polypeptide factor of claim 1, which has homology with metal ion efflux proteins from other eucaryotic species.
 24. The polypeptide factor of claim 1, which confers on E. coli increased resistance to heavy metal ions. 